1. Field of the Invention
The present invention relates to optical discs, and more particularly to an optical disc for recording and reproducing data by a laser beam.
2. Description of the Related Art
Conventional counterparts of optical discs are constructed by forming a recording film 21 on one surface of a plastics substrate 20 and covering the recording film 21 with a protection film 22 as shown in FIG. 6.
Since the recording film 21 generally consists of four or three layers, moisture will not permeate through the recording film 21 to the other side of the plastics substrate 20. Consequently, these optical discs tend to absorb or desorb moisture only at the other side of the plastics substrate 20, i.e. at the incident side. Moisture absorption and desorption was caused by a partial variation in volume of the plastics substrate 20 to generate a warp thereof.
The above warp of the optical discs cause the substrate surface to shift from the position perpendicular to optical axis of the laser beam incident for recording and reproducing data with the result that the incident light fails to strike signals, thereby causing the servo control to get disordered to degrade the resulting signal quality or to generate a track jump in servo control in extreme cases.
Under these circumstances, there have been proposed standards or draft standards with regard to the warp of optical discs as shown in Table 1.
TABLE 1 ______________________________________ Type of optical discs Level of warp ______________________________________ Compact disc 10 mrad max. Write-once disc 5 mrad max. Magneto-optic disc 5 mrad max. ______________________________________
Compact discs have a relatively slow rotating speed practically ranging from 200 to 500 rpm, which can be sufficiently tracked by the tracking servo and the focus servo. Thus, another mild standard on warp is applied to the optical discs in comparison with the counterpart applied to other types of optical discs.
Such discs as write-once discs and rewritable discs (magneto-optic discs) require an increase in the data transfer rate and hence must rotate at a high speed (1800 to 3600 rpm for example). For such discs, the warp level needs to be reduced as much as possible to meet with the tracking capabilities of the tracking servo and the focus servo. However, a single plate plastics substrate hardly reduces the warp level. Thus, single-plate discs have been laminated to each other back-to-back to provide a double-sided disc, thereby reducing the warp level.
However, the overwriting technique has come to capture much attention recently to increase a demand for single-sided discs. That is because conventional magneto-optic discs involve the step of erasing old data prior to recording new data for rewriting data. Consequently, they need two rotations in total to rewrite data; the first rotation for erasing old data and the second rotation for recording new data. In contrast, the overwriting-technique permits erasing old data and recording new data in one rotation to obviate the conventional process of two rotations, thereby increasing the data transfer rate.
There have been proposed so far several methods for overwriting, among which the magnetic field modulation method is one of the most promising ones. The magnetic field modulation method involves recording data by shifting the direction of the magnetic field while continuing the application of light onto the surface of optical discs whereas a data erasing dependent method (optical modulation method) involves recording data by intermittently applying light with the magnetic field kept to a certain direction.
The magnetic field modulation method demands shifting the direction of the magnetic field at a high speed. Besides, actualizing a high-speed magnetic field modulation by minimizing the consumption of electric power in the electromagnet requires a minimization of the distance between the electromagnets and the recording film. However, with respect to double-sides discs, the presence of the substrate on the recording film as viewed from the side provided with an electromagnet inhibits the reduction of the distance between the recording film and the electromagnet, which has created an increased demand for single-side discs.
Using a plastics substrate for single plate discs will naturally compel users to meet with the possibility of the warp of discs. The warp generated immediately after the manufacturing process can be suppressed to a level within the scope of the standard because of the advancement of the technique for forming the substrate, as well as the technique for forming the recording film and the protection film. On the other hand, it has been made clear that another kind of a warp has been generated in using discs. For example, the 35th Applied Physics Association Drafts, in spring 1988, describes on page 872 that it has been made clear that a change in the environmental conditions generates a transient warp of discs. For example, according to the document, a change in relative humidity from 90% R. H. to 50% R. H. at 60% has changed the warp by 14 to 15 mrad at most.